When you are offered a "rate lock" from the lender, it means that you are guaranteed to keep a specific interest rate for a determined period while you work on your application process. This keeps you from getting through your whole application process and learning at the end that your interest rate has risen higher.
While there can be a choice of rate lock periods (from 15 to 60 days), the extended ones are usually more expensive. You can get a longer period for your lock, but in doing so, will most likely have a higher rate than you would have with a shorter rate lock period
There are other ways to get a lower rate, in addition to opting for a shorter rate lock period. A bigger down payment will give you a better interest rate, since you'll have more equity from the beginning. You can pay points to reduce your rate for the loan term, meaning you pay more initially. To a lot of people, this makes financial sense..
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